Grace, written and directed by James Brennan. Designed by Adam Gardnir, lighting by Nik Pajanti, sound design by Peter Brennan. With Gary Abrahams, Brian Lipson, Katrina Miilosevic, Luke Mullins, Ivan Thorley and Carla Yamine. GoD BE IN MY MouTH @ Theatreworks until March 25.

Grace is an illustration of how theatre is always more than the sum of its parts. It has a stellar cast, arresting design, ambitious themes, inventive staging... what's not to like? But the fact is that these elements remain merely elements, never fusing into that indefinable whole that makes absorbing theatre.

It's a rather interesting failure, all the same. It seems to me that most of its limitations originate from the text. Grace appears to be a conventional play that wants to be a spectacle - Ionesco with his brain fried on acid perhaps, or Arabal on sedatives - and somehow falls between two stools, neither satisfying the desire for anarchy or strange beauty on the one hand, nor for rational critique on the other.

The play only makes sense if you read it as an allegory (psychologically speaking, it makes no sense at all): it is a metaphysical, rather than domestic, drama. But its symbology is too simplistic to satisfactorily explore contemporary spiritual desolation, which I think is its intention.

Grace is about the meeting between estranged 14-year-old twins Wade (Luke Mullins) and Serbia (Katrina Milosevic), who have been in foster homes for ten years, after the recent death of their father. They meet on the roof of a city building, the home of their rather sinister Uncle (Brian Lipson) and various giant pigeons (Gary Abrahams, Ivan Thorley and Carla Yamine). Wade, a polite public schoolboy with sadistic homicidal fantasies, represents the human yearning for meaning and spiritual significance; Serbia is materialistic desire, the brutalised human soul who has learned, through bitter experience, only to trust what she has in her hand.

The twins' Uncle is an unshaven, solitary bum who is more interested his angelic pigeons than in his nephew and niece. He's presumably the God we're left with after the death of God the Father, a dethroned patriarch withdrawn into his declasse heaven. And the twins are there to demand their inheritance, left to them by their father, although they have differing desires: Wade wants a reunited family, or perhaps simply to murder them; Serbia just wants lots of money, in lieu of a happiness she yearns for but no longer believes possible. Instead, the Uncle bolts their legs together, leaving the two warring divisions of the human soul to bleed to death. In the face of this meaningless injustice, all that remains to them is to face their deaths with grace.

The twins are presumably a kind of syzygy, the Gnostic male/female twinning who together represent the totality of the Divine; but the first thing I thought of (especially when the sinister Uncle appeared) was the Baudelaire twins in Lemony Snicket's A Series of Unfortunate Events. This may be deliberate - the show is, after all, billed as a comedy - or just an unfortunate quirk of my brain; in any case, it strikes me that Snicket's adventures are a more successful satire on faith than Brennan's, if only because of their cynically circumscribed vision. Invoking the sacred in theatre is a much more difficult business. And here, except in brief, tantalising moments, Brennan achieves oddness rather than the frisson of authentic strangeness.

There is not much of a clue of what Brennan actually means by grace, although everything around the play - the name of the company, the title of the play, the holy music that drifts over the stage, even the eye-catching poster, an avian take on Michelangelo's Pieta - announces that we are in the vicinity of Christian iconography.

So I consulted the Divine Doctor, Thomas Aquinas himself, who has a thing or two to say about Grace (and who may have some connection with Brennan's Uncle, his having been a doctor in his former life). In the gravely seductive clarity of Aquinas' argument, Grace is a most complex thing. It is the presence of God in human beings through the Holy Spirit, the "External Principal of Human Actions" that helps human beings "to act rightly". Without Grace, we can know nothing: not love, nor truth, nor goodness. And most certainly, we can't know God.

Brennan's question seems less to do with whether grace is possible in a Godless world, and more with whether a degraded God can offer grace. Which presupposes the existence of God in the first place, whether a first-order patriarch or a second-order street bum with some kind of hidden bank account. Whatever the question is, it seems deeply problematic if the theatre itself can't attain its own definition of the sacred.

On the surface, a concept dear to mediaeval Christian philosophy seems an unlikely preoccupation for an innovative theatre maker, but avant garde theatre from Artaud to Brook has a rich history of obsession with the sacred, chiefly with mystical traditions. In thinking about Grace, I find myself at a loss: Brennan's production - with its striking images of pigeon-headed dancers and its caged God - seems to be gesturing towards this avant garde tradition, but at the same time invokes an intellectual tradition that is notable for its lucent rationality (whatever one thinks about a system of thought that depends for its existence on a myth, the fact remains that Aquinas' logical arguments are, well, logical). And somehow neither of these opposing impulses are worked out enough or extreme enough to create an interesting dialectic: it makes neither enough sense nor enough nonsense.

This basic conceptual fuzziness may explain why the show leaves little aftertaste, although in its staging it sometimes approaches moments of imaginative irrationality that create a feeling of theatrical event: pigeons dancing to the Bangles' Eternal Flame, or a great speech in which Brian Lipson - always a compelling presence on stage - lists all his losses (his job, his money, his parking space for his car). Luke Mullins and Katrina Milosevic seem rather at a loss in their performances, making mannered gestures towards psychological ideas of character that are pulled up short by the allegorical nature of their roles.

It's hard to escape the feeling that Brennan, whose previous work has been site-specific, has to face different kinds of challenges by staging his work in a conventional theatre: among them, what he wants his spectacle to be. It's a very mixed bag from GoD BE IN MY MouTH but, as I said, it makes a most intriguing failure.